The Taenia ovis tapeworm exists in adult form in the small intestine of its primary host, the dog. The cystic stage is carried in the musculature of its secondary or intermediate hosts, notably sheep and goats. Current control measures include prevention of feeding of infected carcases to dogs and treatment of dogs with cestocidal drugs, notably praziquantel (Droncit, Bayer) to prevent transmission of the parasite to ruminants. These control measures are costly to implement and are not effective in eradicating T.ovis.
Accordingly, as an adjunct to current control measures and to effect eradication of the disease, it would be preferable to immunise the secondary hosts to protect them from infection and also to preserve carcase quality for the meat industry.
Previous investigations conducted into vaccination against T.ovis infection with oncosphere antigens are reviewed by Rickard, M. D. and Williams, J. F, Hydatidosis/Cystercercosis: Immune mechanisms and Immunisation against infection, Adv Parasitology 21, 230-296 (1982). However, in the work reviewed no attempt was made to identify which antigenic component of the oncospheres was responsible for the immune response. As will be appreciated, T.ovis contains a large number of antigenic components, most of which are not immunologically effective against infection.
Earlier attempts have been made to identify a host protective antigen for T.ovis (Howell, M. J & Hargreaves, J J Mol Biochem Parasitol 28, 21-30 (1988)). A cDNA library was prepared using mRNA extracted from adult T. ovis tape worms. Recombinants expressing antigenic determinants as .beta.-galactosidase fusion proteins were selected using antibodies in serum from sheep infected with T. ovis. Some fusion proteins were shown to correspond with native antigens (92.5 to 180 kD) present in adult and oncosphere stages of T.ovis, but trials of the host-protective nature of purified fusion proteins were not reported.
In Johnson, K. S. et al, Nature 338, 585-587 (1989), the present inventors have described the identification and cloning of a native polypeptide of T.ovis capable of generating a protective immunological response in ruminants against T.ovis infection. However, the recombinant polypeptide described in this paper has been found to be less stable than is optimal for the production of a commercial vaccine.
It is accordingly an object of the present invention to provide a stable form of protective antigen for use in vaccines for the protection of ruminants against T.ovis infection or at least to provide the public with a useful choice.